The kraft pulping process is generally the most widely used chemical pulping process employed in the pulp and paper industries. In this process, wood chips or other fibrous materials are heated under pressure with steam in a sodium hydroxide and sodium sulfide solution. The resulting pulp is generally used to make brown paper. It can also be bleached to produce white pulp for writing and other higher grade papers.
The filtrate and washings from the pulping process--the so-called "black liquor"--generally contain dissolved organic materials from the wood as well as spent pulping chemicals. The black liquor is normally concentrated in evaporators and then burned under both oxidizing and reducing conditions in a boiler or combustor. The combustion of the organic matter contained in the black liquor produces high pressure steam which can be used to generate electricity. The inorganic materials form a molten kraft smelt on the bottom of the combustor. The typical kraft smelt primarily contains sodium and potassium carbonate, sodium sulfide, and sodium sulfate.
Molten kraft smelt, generally at a temperature of about 750.degree. to 950.degree. C., is normally removed from the bottom of the boiler via water-cooled troughs (the so-called smelt spouts) and fed directly into an aqueous solution contained in a dissolving tank. Thus, the kraft smelt is fed into the dissolving tank while still in the molten state. The kraft smelt dissolves in the aqueous solution and produces a so-called "green liquor". The green liquor, after filtering or clarifying to remove undissolved materials, is subjected to a recausticizing operation. The green liquor is treated with lime to convert the sodium carbonate into sodium hydroxide through slaking and causticizing reactions. In the slaking reaction, calcium oxide is converted to calcium hydroxide; in the causticizing reaction, the calcium hydroxide reacts with sodium carbonate to produce calcium carbonate and sodium hydroxide. This lime-treatment process produces kraft "white liquor" containing the pulping chemicals which can then be reused or recycled in the kraft pulping system.
The recausticizing operation for kraft pulping also involves a reaction train, known as the calcium cycle, to replace CO.sub.3.sup.2- in the green liquor with OH.sup.- needed for kraft cook. CaO is reacted with the Na.sub.2 CO.sub.3 in the green liquor to form CaCO.sub.3, which is separated. The CaCO.sub.3 is re-burned to form CaO in a lime kiln, and carbon is released as CO.sub.2. This closes the calcium cycle.
One of the largest industrial processes involving inorganic chemistry in the U.S. is the recovery of inorganic pulping chemicals for the kraft process in the pulp and paper industries. One major part of this process is recausticizing. This chemical recovery system forms an important and vital part of the paper-making process. Indeed, without the chemical recovery system, the kraft system would be prohibitively expensive. Moreover, the disposal of the black liquor would likely be impossible in an environmentally acceptable manner.
This chemical recovery process or system has, however, a number of problems and/or limitations. For example, feeding molten kraft smelt (normally about 750.degree. C. or higher) directly into an aqueous solution can result in, and often has resulted in, explosions in the dissolving tank. Such explosions, in addition to potentially damaging equipment and injuring personnel, can release significant amounts of sulfur-containing pollutants into the environment, especially into the atmosphere. Also, it is difficult to control the green liquor concentration when feeding molten kraft smelt. Finally, the lime cycle poses many problems. Chemical equilibrium limits the conversion of carbonate. The operation of the lime kiln has significant environmental impact as a result of gaseous and particulate emissions. Fossil fuels are burned in the kiln in an inefficient and hard-to-control operation. In low effluent pulp production, enrichment of elements such as Mg, Al, and P can lead to serious disturbances in the lime cycle. Large amounts of non-process elements are introduced into the pulping operation from replacement lime.
Moreover, the production of many current mills is significantly limited by the recausticizing operation. Incremental capacity cannot easily be installed, due to the nature of the calcium/lime cycle. In addition, many mills are limited in the space available for equipment.
Considerable efforts have been made to improve the existing kraft recovery process or to provide alternative kraft recovery processes. See, for example, Nishizawa et al., "Chemical Recovery Process by Direct Carbonation of Smelt," Proc. IUPAC/EUCEPA Symposium on Recovery of Pulping Chemicals (Helsinki) 659-73 (1968); Grace, "Gasification: Route to the Promised Land?'", 70 PIMA 75-76 (1988); Empie, "Alternative Kraft Recovery Processes," 74 Tappi J. 272-76 (1991); DeNovo et al., U.S. Pat. No. 4,303,496 (Dec. 1, 1981); Empie, U.S. Pat. No. 4,441,959 (Apr. 10, 1984); Feldmann, U.S. Pat. No. 4,522,685 (Jun. 11, 1985); and Empie, U.S. Pat. No. 4,526,760 (Jul. 2, 1985). Although the overall process has been improved in a number of ways, the kraft recovery process generally employed today still involves feeding molten kraft smelt directly into an aqueous solution in the dissolving tank to produce green liquor followed by further physical and chemical treatment to produce white liquor.
It would be desirable, therefore, to provide a kraft recovery system which eliminates the steps of producing green liquor and treating the green liquor to produce white liquor. Such a system would avoid the process control difficulties inherent in these steps. It would be desirable to provide a kraft recovery system that eliminates the entire recausticizing operation, with its many pollution sources, ill-defined process steps and low energy efficiency. It would also be desirable to provide a kraft recovery system with significantly increased energy efficiencies, limited sulfurous emissions and minimal solids handling. The present invention provides such a kraft recovery system. Moreover, the kraft recovery system provided by the present invention can be incorporated into existing kraft recovery systems with relative ease.